Present distribution and abundance of the introduced barnacle Balanus glandula Darwin in South Africa

The date of first introduction of the North-East Pacific acorn barnacle Balanus glandula to South Africa is unknown, but it is depicted in photographic records dating back to at least 1992. Its present range and population density were determined by surveying 24 sites on the west and south coasts of South Africa. B. glandula occurred over approximately 400 km of coastline, from Elands Bay in the north-west to Misty Cliffs on the west coast of the Cape Peninsula. The abundance of B. glandula was affected by site, zone, prevalence of upwelling, and orientation of the shore. Its distribution was very patchy, both on macro- and microscales, with Bloubergstrand and Moullie Point having the highest abundances of 28 445 and 24 500 individuals m(-2) respectively. B. glandula was the dominant barnacle at all West Coast sites where it was recorded, comprising 78.5% of all barnacles found. The successful invasions of B. glandula in South Africa, Argentina and Japan suggest that this species poses a potential threat to intertidal communities in cool, temperate waters.